This invention relates to thermostats for electric heating or cooling apparatus. More particularly, the invention is concerned with so-called line voltage thermostats, wherein the load current required by an electric heater or a room air conditioner is carried by the switch that forms part of the thermostat.
The switching means that forms part of line voltage thermostats is proportioned to carry and to switch relatively heavy load current. The line current supplied to a room electric heater (for example) is switched on and off by switching means that forms part of a line voltage thermostat. This contrasts with the low levels of current carried by the switch in a thermostat designed for controlling a relay (so-called "low-voltage" thermostats) wherein it is the relay that carries and switches the load current. In low-voltage thermostats, the switch in the thermostat does not become heated as a result of the small current that it carries, so that internal self-heating is not a problem in low-voltage thermostats.
The conduction of relatively heavy current by the switch of a line-voltage thermostat produces a significant amount of heat. This heat tends to modify the temperature in the ambient temperature sensing zone of the thermostat. The effect is small in mild weather, when the switch is closed for only short periods, with relatively long intervening periods when the switch is open. On the other hand, when an electric room heater (or air conditioner) is called upon to operate for long periods with only short interruptions, then a substantial amount of heat is produced by the current conducted by the thermostat switch. The temperature in the ambient-temperature measuring zone is raised significantly by this heat. The temperature sensing element "sees" a higher temperature than actually prevails in the room and, accordingly, tends to control the temperature modifying apparatus of the room so as to establish a lower temperature in the room than that for which the thermostat is set. This effect is called "droop". Droop has been a persistent, troublesome characteristic of line-voltage thermostats.
One approach to providing a line-voltage thermostat having a controlled droop characteristic or characterized by a minimum or even a zero droop is disclosed and claimed in U.S. Pat. No. 3,553,624 for an Anti-Droop Thermostat. In that patent, a supplementary heater is incorporated in a line-voltage thermostat to develop heat during the time intervals when the switch is off and when the load current that develops heat in the switch is consequently interrupted. To perform in the intended manner, the resistor in that thermostat should be selected to produce an amount of heat that matches the heat produced in the switch by the load current. Other line-voltage thermostat arrangements have been developed or proposed that are relatively complicated and expensive, involving the use of compensating elements arranged to act between the main bimetal of the thermostat and the thermostat switch or within the switch itself, for meeting the problem of thermostat droop.
Thermal cycle-timing resistors that are incorporated in thermostats have the effect of inducing the thermostat to cycle on-and-off five or more times per hour. When cycle-timing heating elements are not used, the thermostat responds slowly to the effects of the room heater with the result that the room temperature tends to overshoot excessively above and below the temperature setting of the thermostat. Known thermostats incorporating thermal cycle-timing involve various problems and complications in connection with the location and the mounting of the heating resistor.